There is a desire to be able to frequency synthesize high precision optical signals over many decades of bandwidth with a high frequency stability (˜1 Hz) for LADAR, remote sensing, and optical spectroscopy, among other uses, including automotive and aerospace applications.
A very wide tuning range (1 Hz-10 GHz) is desirable, but the ability to obtain such a large range is currently limited by the dynamic range of the available electronic components. FIGS. 1a and 1b show conventional methods for providing digital optical path changes in prior art using transverse electro-optic phase modulators. FIG. 1a is a block diagram illustrating the apparatus used in a conventional method of providing analog controlled optical path changes a length and FIG. 1b is a block diagram illustrating the apparatus used in a conventional method of digital controlled optical path length changes. In FIG. 1a, an optical input signal is coupled to an input of an optical delay device 102 in such a modulator. An optical output of the optical delay device 102 produces a delayed optical output signal. A driver amplifier 104 is coupled to a control input terminal of the delay circuit 102, and is responsive to an analog control signal. In operation, the optical delay device 102 modifies the optical beam to be delayed relative to the input signal by an amount specified by the value of the analog control signal. In general the optical beam can propagate bidirectionally that is from I/O 101-1 to I/O 101-2 or from I/O 101-2 to I/O 010-1.
In FIG. 1b, an optical input signal is coupled to an input terminal of a similar delay device 102. An output terminal of the delay circuit 102 of FIG. 1b produces a delayed optical output signal. A digital-to-analog converter (DAC) 154 is coupled to a control input terminal of the delay circuit 102, and is responsive to a multi-bit digital control signal. In operation, the digital-to-analog converter 102 converts the multi-bit digital control signal to a corresponding analog control signal. The analog control signal is coupled to the control input terminal of the optical delay circuit device 102. The delay device 102 produces an output signal which is delayed relative to the input signal by an amount specified by the value of the digital control signal. In general the optical beam can propagate bidirectionally that is from I/O 102-1 to I/O 102-2 or from I/O 102-2 to I/O 102-1 in delay device 102.
In FIG. 1b, an optical input signal is coupled to an input terminal of a similar delay device 102. An output terminal of the delay circuit 102 of FIG. 1b produces a delayed optical output signal. A digital-to-analog converter (DAC) 154 is coupled to a control input terminal of the delay circuit 102, and is responsive to a multi-bit digital control signal. In operation, the digital-to-analog converter 102 converts the multi-bit digital control signal to a corresponding analog control signal. The analog control signal is coupled to the control input terminal of the optical delay circuit device 102. The delay device 102 produces an output signal which is delayed relative to the input signal by an amount specified by the value of the digital control signal. In general the optical beam can propagate bidirectionally that is from I/O 102-1 to I/O 102-2 or from I/O 102-2 to I/O 102-1 in delay device 102.
In FIGS. 1a and 1b, the resolution is limited by the electrical properties of the analog driver amplifier 104 and/or the DAC 154. Specifically, the maximum output voltage from the analog driver amplifier and/or DAC sets the most significant optical change and the point at which the noise level becomes significant sets the minimal output level. With existing DACs, the dynamic range is on the order of 12-20 bits depending on the output bandwidth.
On the other hand, at least 34 bits are required to define a frequency tuning range of 10 GHz with 1 Hz resolution. An optical device which can augment a presently achievable DAC resolution of 20 bits by an additional 10 to 20 bits with a comparable noise level is therefore desirable.
The current invention permits a designer to circumvent the dynamic range limitation of current DACs by augmenting it with weighted optical elements for the most significant bits each driven by a single binary electrical line of multi-bit bus. Each of the more significant bits may operate with the same digital driver voltage well above the electrical noise level of the driver. At that point when the resolution of the chain is limited by the optical element, the less significant bits may be driven by a multi-bit conventional DAC to provide fine resolution.
One advantage of this approach is realized when each binary element (individual modulator), has a high impedance and is driven by a fully complimentary CMOS driver with approximately zero static current. Since MOSFET 1/f noise is normally proportional to the current, the 1/f noise of approximately zero current CMOS drivers will be negligible. So CMOS drivers are preferable compared to either traditional (non-complementary) MOS or bipolar drivers.